A) Field of the Invention
The present invention relates to a semiconductor device, its manufacture method and an electronic component unit.
B) Description of the Related Art
Semiconductor light emitting devices have been proposed which have the structure that a semiconductor light emitting layer is bonded to a conductive substrate (for example, refer to Japanese Patent Laid-open Publication No. 2001-189490, No. 2001-44491, No. 2002-217450, No. HEI-5-251739, and U.S. Pat. No. 5,917,202, which are incorporated herein by reference).
FIG. 19 is a schematic cross sectional view showing an example of a conventional semiconductor light emitting device 61 having the structure that a semiconductor light emitting layer is bonded to a conductive substrate. A reflection layer 68 made of metal is stacked on a conductive support substrate 63, e.g., a Si substrate heavily doped with n-type impurities. Epitaxially grown on this reflection layer 68 are an n-type clad layer 66 having a potential barrier function of holes, an active layer 65 for emitting light upon recombination of holes and electrons and a p-type clad layer 64 having a potential barrier function of electrons, in this order from the bottom. On the p-type clad layer 64, a p-side ohmic electrode 62 is formed. An n-side optical output electrode 67 is formed on the conductive support substrate 63 on the side opposite to the reflection layer 68.
Light generated in the active layer 65 and became incident upon the reflection layer 68 before reaching the conductive support substrate 63 is reflected by the reflection layer 68 and output from the semiconductor light emitting device 61. If the reflection layer 68 is made by reducing an angle dependency of a reflectivity, an optical output efficiency can be improved.
FIG. 20A to 20D are schematic diagrams illustrating a manufacture method for the semiconductor light emitting device 61 shown in FIG. 19.
Reference is made to FIG. 20A. A reflection layer 68 is formed on a conductive support substrate 63 to form a first substrate 70.
Reference is made to FIG. 20B. On a temporary substrate (growth substrate) 69 made of, e.g., GaAs, a p-type clad layer 64, an active layer 65 and an n-type clad layer 66 are epitaxially grown in this order from the bottom, to form a second substrate 71.
Reference is made to FIG. 20C. The first substrate 70 shown in FIG. 20A is bonded to the second substrate 71 shown in FIG. 20B, with the metal layer 68 being adhered to the n-type clad layer 66.
Reference is made to FIG. 20D. After the temporary substrate 69 is removed, a p-side ohmic electrode 62 is formed on the p-type clad layer 64, and an n-side optical output electrode 67 is formed on the conductive support substrate 63 on the side opposite to the reflection layer 68.
In a device having the bonding structure, ohmic electrodes are formed after a bonding process. Since the device is heated to a higher temperature (about 400° C. to 500° C.) than a bonding temperature in order to form ohmic contacts, a bonding layer made of bonding eutectic material or solder is again heated and melted so that peeling (include “floating” not complete peeling) may occur. The peeling becomes the reason of lowering reliability.
The reflection layer 68 has an ohmic contact with the n-type clad layer 66, providing an n-side electrode function. There is a severe trade-off between good reflection characteristics and good ohmic contact. In order to form an ohmic contact, an alloying process is necessary. However, during alloying, electrode material diffuses and a reflectivity is lowered.
Solder or eutectic material (not shown) is used to adhere (bond) the first substrate 70 and second substrate 71. If solder or eutectic material enters the reflection layer, the reflection characteristics of the reflection layer are degraded. In addition to this problem, there is another problem of ball-up of solder or eutectic material on the substrate 70 if the substrate 70 is larger than the substrate 71, when the two substrates 70 and 71 are bonded together.
U.S. Pat. No. 5,917,202 proposes the structure of a semiconductor light emitting device in which a barrier layer and a solder bonding layer are disposed between a reflection layer and a solder layer. The barrier layer is made of tungsten (W) or molybdenum (Mo) and prevents element diffusion, and the solder bonding layer is made of nickel (Ni) and improves tight adhesion.